There are a variety of electrical system applications which require one or more sources of high voltage AC power. As a non-limiting example, a liquid crystal display (LCD), such as that employed in desktop and laptop computers, or in larger display applications such as large scale television screens, requires an associated set of cold cathode fluorescent lamps (CCFLs) mounted directly behind it for back-lighting purposes. In these and other applications, ignition and continuous operation of the CCFLs require the application of a high AC voltage that can range on the order of several hundred to several thousand volts. Supplying such high voltages to these devices has been customarily accomplished using one of several methodologies.
A first technique involves the use a single-ended drive system, wherein a - high voltage AC voltage generation and control system is transformer-coupled to one/near end of the lamp, while the other/far end of the lamp is connected to ground. This method is undesirable, as it involves the generation of a very high peak AC voltage in the high voltage transformer circuitry feeding the driven end of the lamp.
A second technique involves the use a double-ended drive system, wherein a high voltage AC voltage generation and control system is transformer-coupled to one/near end of the lamp, while connection from the voltage generation and control system to the other/far end of the lamp is effected through high voltage wires. These wires can be relatively long (e.g., four feet or more), making them more expensive than low voltage wires; in addition, they lose substantial energy through capacitive coupling to ground. This method is also very undesirable, as it involves the generation of a very high peak AC voltage in the high voltage transformer circuitry feeding the driven end of the lamp.
Another approach is to place a high voltage transformer and associated voltage switching devices, such as MOSFETs or bipolar transistors, near the far end of the lamp; these devices are connected to and controlled by a local controller at the near end of the lamp. This method has disadvantages similar to the first, in that the gate (or base) drive wires are required to carry high peak currents and must change states at high switching speeds for efficient operation. The long wires required are not readily suited for these switching speeds, due their inherent inductance; in addition they lose energy because of their substantial resistance.
Pursuant to the invention disclosed in the above-referenced '976 application, these and other disadvantages of conventional high voltage AC power supply system architectures, including systems for supplying AC power to CCFLs used to back-light an LCD panel, are effectively obviated by means of a double-ended, DC-AC converter architecture, which is operative to drive opposite ends of a load, such as a CCFL, with a first and second sinusoidal voltages having the same frequency and amplitude, but having a controlled phase difference therebetween. By controlling the phase difference between the first and second sinusoidal voltages, it is possible to control the amplitude of the composite voltage differential produced across the opposite ends of the load.
In accordance with a first, voltage-driven, push-pull embodiment, the invention disclosed in the '976 application is implemented by means of first and second, voltage-fed, push-pull DC-AC converter stages having respective output ports coupled to opposite ends of the load (CCFL). Each push-pull converter stage contains a pair of pulse generators which produce phase-complementary rectangular wave pulse signals of the same amplitude and frequency having a 50% duty cycle. These phase-complementary pulse signals are used to control the ON/OFF conduction of a pair of controlled switching devices, such as respective MOSFETs, whose source-drain paths are coupled between a reference voltage terminal (e.g., ground) and opposite ends of a center-tapped primary coil of a step-up transformer. The center tap of the primary coil of the step-up transformer is coupled to a DC voltage source, which serves as the DC voltage feed for that DC-AC converter stage. The secondary coil of the step-up transformer has a first end coupled to a reference voltage (e.g., ground) and a second end coupled by way of an RLC output filter to one of the two output ports. The RLC circuit converts the generally rectangular wave output produced across the secondary winding of the step-up transformer into a generally sinusoidal waveform.
In operation, the complementary phase, rectangular waveform, 50% duty cycle output pulse trains produced by the two pulse generators alternately turn the two MOSFETs ON and OFF, in a mutually complementary manner. Whichever MOSFET is turned on will provide a current flow path to ground from the voltage source feed through half of the center tapped primary winding and the drain-source path of that MOSFET. The alternating of the conduction cycles of the two MOSFETs of a respective converter stage has the effect of producing a generally rectangular output pulse waveform having a 50% duty cycle across the secondary winding of the step-up transformer for that stage. The amplitude of this voltage waveform corresponds to the product of the secondary: primary turns ratio of the transformer and twice the value of the DC voltage of the voltage feed source. The shape of this generally rectangular waveform is converted by the RLC filter into a relatively well defined sinusoidal waveform, that is supplied to one of the two output ports and thereby to one end of the load (CCFL).
The controlled phase shift mechanism serves to controllably shift the phase of the sinusoidal waveform produced by the output RLC filter of one of the converter stages by a prescribed amount relative to the phase of the sinusoidal waveform produced by the output RLC filter of the other converter stage. This controlled imparting of a differential phase shift between the sinusoidal waveforms appearing at the two output ports has the effect of modifying the shape and thereby the amplitude of the composite AC signal produced between the two output ports.
Producing the incremental phase offsets between the two waveforms generated by the two converter stages may be readily accomplished by imparting a controlled amount of delay to the pulse trains produced by the pulse generators of one of the converter stages relative to the pulse trains produced by pulse generators of the other converter stage. The amount of delay between the two pulse trains will control the shape and thereby the amplitude of the composite AC waveform produced across the output ports.
A second, current-fed embodiment of the invention disclosed in the '976 application comprises first and second, current-fed, push-pull DC-AC converter stages respective output ports of which are coupled to opposite ends of a load such as a CCFL, as in the first embodiment. As in the first embodiment, the current-fed, double ended push-pull, DC-AC converter stages are operative to produce first and second sinusoidal voltages having the same frequency and amplitude, but having a controlled phase difference therebetween, which is effective to modulate the amplitude of the composite AC voltage produced across the opposite ends of the load.
As in the first embodiment, each current-fed, converter stage has a pair of complementary pulse generators, which produce phase-complementary rectangular output pulse signals having a 50% duty cycle. Each rectangular wave signal is applied to the control terminal of a controlled switching device, such a controlled relay, which is operative to controllably interrupt a current flow path therethrough coupled between a prescribed reference voltage (e.g., ground) and one end of a parallel connection of a capacitor and a center-fed primary winding of a step-up transformer, which form a resonant tank circuit, that serves to deliver a resonant sinusoidal waveform of a fixed frequency and amplitude to the secondary winding of the transformer. The primary winding of the step-up transformer has its center tap coupled through a resistor and an inductor to a DC voltage source, which serves as the current feed for that converter stage.
In operation, the complementary phase, rectangular waveform 50% duty cycle output pulse trains produced by the pair of pulse generators alternately close and open the controlled switches in a complementary manner. Whenever a switch is closed, a current flow path is established from the battery terminal though an inductor and resistor to the center tap of the transformer's primary winding, and therefrom through half of the primary winding, a resistor and the closed current flow path through the switch to ground. A prescribed time after the closure of one switch and the opening of the other switch, the states of the two pulse signal inputs to the control inputs of switches are reversed. Due to the inherent inertia property of the transformer's primary winding, current therethrough does not immediately cease flowing. Instead, current from the primary winding flows into one side of the capacitor connected in parallel with the primary winding.
The resonant circuit formed by the capacitor and the primary of the step-up transformer results in a ringing of the current between the capacitor and the primary winding of the transformer, which serves to induce a sinusoidal waveform across the secondary winding. The waveform on one side of the resonant tank capacitor is a one-half positive polarity sine wave, while the waveform on the other side of the capacitor is a one-half negative polarity sine-wave. The resultant of the two one-half sine waves, which is applied to one of the output ports, is a sine wave of fixed amplitude, frequency and phase.
In order to controllably shift the phase of the resultant sine wave supplied to the one output port relative to the other output port, transitions in the complementary 50% duty cycle pulse trains produced by the pulse generators of one converter stage are incrementally delayed with respect to the pulse trains produced by the pulse generators of the other stage, so as to controllably shift the phase of the sine wave supplied to the one output port relative to the other output port. As in the voltage-fed embodiment, incrementally offsetting in phase of the two sine waveforms produced by the push-pull DC-AC converter stages of the current-fed embodiment serves to vary or modulate the amplitude of the composite waveform produced across the two output terminals.
A voltage controlled delay circuit is used to define the relative delay between the complementary pulse trains that are applied to the pulse generators within the respective push-pull DC-AC converter stages of the embodiments of the invention, and thereby control the amplitude of the composite AC waveform produced across the driven load. Incrementally varying the magnitude of the DC voltage applied to the voltage control input serves to controllably adjust the delay between the transitions in the complementary 50% duty cycle pulse trains produced by one pair of pulse generators with respect to the pulse trains produced by the other pair of pulse generators, so as to controllably shift the phase of the resultant sine wave supplied to one output port relative to the sine wave applied to the other output port. This serves to modulate the amplitude of the composite AC voltage produced across the opposite ends of the load.